[FIX] web_editor: prevent mismatch between mimetype and content type

Since the image util method `image_base64()` is converting BMP images to PNG
automatically[1], there would be a mismatch between the attachment content type
and the attachment mimetype.

  - Content type: retrieve with `base64_to_image(data).format` -> return PNG as
                  the image data was converted from BMP to PNG
  - Mimetype: set on `write()` and `create()` with `_check_contents()` which
              call `mimetypes.guess_type(file_name)`.
              BMP image `my_file_image.bmp` would have a mimetype set to BMP

The computed field `image_src` would return `False` since the image.mimetype
would be BMP, making the dialog picker not showing the image but loading
`http://localhost:8069/false` instead.

[1] https://github.com/odoo/odoo/blame/097f29d0f6a5b295c81fd074665dbd261eaf12fb/odoo/tools/image.py#L124

task-2345082
Coming from #65828
Closes #68326

X-original-commit: ef5b108b781df73342bb99342a1116fad86d3b58

addons/web_editor/controllers/main.py

@@ -243,6 +243,10 @@ class Web_Editor(http.Controller):
 
     def _attachment_create(self, name='', data=False, url=False, res_id=False, res_model='ir.ui.view'):
         """Create and return a new attachment."""
+        if name.lower().endswith('.bmp'):
+            # Avoid mismatch between content type and mimetype, see commit msg
+            name = name[:-4]
+
         if not name and url:
             name = url.split("/").pop()